Abstract

The nuclear root-mean-square charge radius of Ni54 was determined with collinear laser spectroscopy to be R(Ni54)=3.737(3) fm. In conjunction with the known radius of the mirror nucleus Fe54, the difference of the charge radii was extracted as ΔRch=0.049(4) fm. Based on the correlation between ΔRch and the slope of the symmetry energy at nuclear saturation density (L), we deduced 21≤L≤88 MeV. The present result is consistent with the L from the binary neutron star merger GW170817, favoring a soft neutron matter EOS, and barely consistent with the PREX-2 result within 1σ error bands. Our result indicates the neutron-skin thickness of Ca48 as 0.15–0.21 fm.